The present invention is directed toward an improved tonometer system for use in the early detection of glaucoma. The tonometer of the present invention provides for an accurate measurement of intraocular pressure without making any physical contact with the eye and without need for eye drops or anesthetic. The tonometer of the present invention achieves these measurements through non-invasive and non-contact techniques. The tonometer of the present invention is related to and an improvement upon the tonometer which is described in my U.S. Pat. No. 4,928,697, which issued on May 29, 1990. The disclosures of my '697 patent are incorporated by reference herein.
Glaucoma is an eye disease which is one of the leading causes of blindness in the U.S. and throughout the world. Glaucoma is fairly common in adults over age 35. Two out of every one hundred persons in this age group have vision threatened by glaucoma.
When an object is viewed, the image is carried from the retina of the eye to the brain by the optic nerve. The optic nerve is an accumulation of over one million individual transmitters, each carrying a message to the brain. The individual messages all join together to provide side vision or peripheral vision as well as sharp central reading vision. Glaucoma can permanently damage the optic nerve, causing blind spots in areas of vision to develop. If glaucoma is undiagnosed, the optic nerve sustains considerable irreversible damage and may even be destroyed, resulting in blindness.
Glaucoma is detectable by measuring the intraocular pressure of the eye. Increased elevations of intraocular pressure are indicative of glaucoma situations or possible glaucoma situations. Intraocular fluid flows through the inner eye continuously to maintain the structure of the eye, in particular, the cornea. If the outflow or drainage system within the eye becomes blocked for any reason, the fluid backs up within the inner eye causing the fluid pressure to increase and thus causing damage to the optic nerve. The possibility of damage to the optic nerve increases with increasing pressure. The only preventative measure which can be taken is the early detection of glaucoma by periodic testing of the intraocular pressure since an elevated intraocular pressure is clearly basic to the whole concept of glaucoma.
A variety of mechanisms have been devised to facilitate the measurement of the intraocular pressure. The common available instrument, known as a tonometer, has the following general operational characteristics. The tonometer measures the force necessary to applanate or flatten a given area of the cornea. The measurement is directly related to the intraocular pressure because the cornea is flattened only when the external force equals the force applied by the ocular pressure plus the force necessary to deform the corneal tissue. The transition of the shape of the cornea from convex to planar can be detected by a simple optical principle. When a reflecting surface is illuminated with a narrow beam of light, the intensity of the reflected light gathered by a converging lens is maximal when the reflective surface is flat. Thus, if the cornea is illuminated by a narrow beam of light, the output of a photodetector placed at the focal plane of the lens will be maximal when the cornea flattens. Further, the flattened area should be ideally located at the apex of the cornea and perpendicular to the optical axis of the eye. This precaution is taken to minimize the deformation of the eyeball and to reduce the subsequent artifical increase of intraocular pressure due to the measurement. It should be noted however that the requirement of proper alignment is not always achieveable and some current tonometers have been designed to compensate for variations in the positioning of the measurement probe with respect to the optical axis of the eye. Finally, the last requirement for most currently available tonometers is that the application of the instrument on the eye be rapid and automatic.
There are an assortment of tonometers currently available which offer a variety of methods for measuring the intraocular pressure of the eye. The basic apparatus is the "Goldmann" applanation-type tonometer which is either hand held or designed for use as a fixed-type instrument. With the applanation-type tonometer, an applanating surface is placed in contact with the cornea and a force applied and varied until a fixed diameter of applanation of the cornea is achieved. The force of application is measured once the fixed diameter of applanation is achieved and this force is used to determine the intraocular pressure of the eye. The applanation-type tonometer must be used with a topical anesthetic. Another type of tonometer is the Schiotz-type or plunger-type tonometer. This tonometer is placed before the eye along the optical axis and a plunger is released which flattens the cornea to a specified diameter and measures the forces applied. Again, the Schiotz-type tonometer requires the use of a topical anesthetic. The Schiotz and Goldmann tonometers, while accurate in their measurement of intraocular pressure are quite undesirable to the patient in that they require the use of a topical anesthetic and further require that the eye be contacted by a mechanical device. Any contact with the corneal tissue carries the risk of infection and corneal abrasion. It also has been found that the patient usually has a somewhat high level of fear and physical discomfort as a result of such eye contact. Thus, the patient will tend to avoid the procedure, if possible.
Another consideration which is currently becoming a priority within the medical community is the desire to insulate the patient, physician and tonometer from mutual contact wherein bodily fluids may be communicated, whether by necessity or inadvertantly. The threat of contacting the Acquired Immune Deficiency Syndrome virus has created a new and desired objective of insulating the patient and eliminating, as much as possible, the need for any physical contact during examination.
A new generation of tonometers have been designed in an effort to limit contact with the eye which utilize an air puff pressure generation on the eye. The air puff system uses an air pulse generated from the ambiant atmosphere with a reliable positive linear force/time ramp. The air pulse impinges upon the cornea causing a gradual curvature reduction, applanation, and a finally slight concavity before the decaying force/time ramp permits restoration. Telecentric optical electronic monitoring of the corneal vertex reflection uniquely identifies, in time, the applanation event. This system has been described in detail in the American Journal of Optometry and Archives of the American Academy of Optometry, Vol. 49, August 1972, No. 8 "A new tonometer system", Grolman. While the air puff type tonometer has eliminated the need for the use of any form of topical anesthetic or the need for contact with the eye, the air puff tonometer creates an audible sound and sends a strong surge of air to the eye. Such sounds and air surges still have the possibility of causing physical or psychological discomfort in some patients.
It is an object of this invention to provide a tonometer which accurately measures the intraocular pressure without need for topical anesthetic and without any discomfort to the patient, whether psychological or physical.
It is another object of this invention to provide a tonometer which accurately measures the intraocular pressure through non-invasive and non-contact type techniques.